The present invention relates to a valve train having a camshaft for actuating intake valves or exhaust valves in an internal combustion engine. More particularly, the present invention pertains to a valve train that actuates a fuel pump by rotation of a camshaft.
In a typical engine, rotational force of a crankshaft is transmitted to camshafts, for example, by a timing belt. The camshafts are rotated, accordingly. Valve cams on the camshafts selectively open and close intake valves and exhaust valves. Fuel injected from fuel injection valves is mixed with air. When the intake valves are opened, the air-fuel mixture is introduced to combustion chambers of the engine. The air-fuel mixture then fills the combustion chambers and is combusted. The combustion of the mixture generates power of the engine. After combustion, exhaust gas is discharged from the combustion chambers when the exhaust valves are opened.
In the above described engine, fuel is pressurized and is supplied to the fuel injection valve by a fuel injection pump. Several types of mechanisms for actuating the fuel injection pump have been proposed (see "Fuel Pump Actuating Mechanism in Engine" disclosed in Japanese Unexamined Utility Model Publication No. 7-22062). In a mechanism of this type, a pump cam is provided on a camshaft for actuating a fuel injection pump. The pump cam contacts a piston of the injection pump thereby converting rotation of the camshaft to reciprocation of the piston. The reciprocation of the piston introduces fuel from a fuel tank into a pressurizing chamber of the pump. The piston then pressurizes the fuel and supplies the fuel to the fuel injection valves.
The torque of a camshaft fluctuates when it selectively opens and closes intake valves or exhaust valves. The intake valves and the exhaust valves are constantly urged by valve springs in a closing direction. When the valves are opened against the force of the springs, torque opposite to the direction of rotating of the camshaft acts on the camshaft. On the other hand, when the valves are closed, torque in the rotating direction of the camshaft acts on the camshaft. These torques fluctuate the torque of the camshaft. Also, the inertia of each valve is another cause of the torque fluctuation in the camshaft.
A fuel injection pump, which is actuated by a camshaft, applies a reactive force on the camshaft. The magnitude of the reactive force during its suction stroke is different from the magnitude during its compression stroke. In other words, the magnitude of the reactive force fluctuates. Therefore, the torque of the camshaft is fluctuated not only by actuation of the intake or exhaust valves, but also by actuation of the fuel injection pump. When the torque fluctuation caused by the intake or exhaust valves and the torque fluctuation caused by the fuel injection pump overlap and are additive, the resultant torque fluctuation in the camshaft results in an excessive tension of the timing belt. This shortens the life of the belt.
Wide torque fluctuation of the camshaft causes the tension of the timing belt to also widely fluctuate. Wide tension fluctuation of the belt vibrates the belt and causes the belt to resonate. The resonance of the belt further increases the tension of the belt. This further shortens the life of the belt.
Some engines use a timing chain or gears to transmit rotational force of a crankshaft to camshafts. In these types of engines, torque fluctuation of camshafts increases the tension of the chain and the load on the teeth of the gears. This shortens the life of the chain or the gears.
Replacing the valve springs with springs having weaker force or changing the cam profile of the intake or exhaust cams will reduce the torque fluctuation of the camshaft caused by actuation of intake or exhaust valves. As a result, the tension of the timing belt will be decreased and resonation of the belt will be prevented. However, weaker valve springs and changed cam profiles degrade the performance (for example, the power) of the engine.